Easy configuration of collector dependent tasks

[sellmair]

e.g.

tasks{
    dokkaJavadocCollector {
        jdkVersion = 13
    }
}

[whyoleg]

Dokka collector tasks were phased out in DGPv2, so this issue is obsolete: https://kotlinlang.org/docs/dokka-migration.html#address-deprecations-and-removals